1. Field of the Invention
The present invention relates to improvement of a see-through type touch sensing device to be used for inputting data displayed at the area on the display screen and is designated by the finger to the computer system and, in more detail, to a touch sensing device which is capable of detecting change of capacitance of the touch sensing electrode occurring by finger touch.
2. Description of the Prior Arts
As a man-machine interface unit which is mounted over the display screen and is used for inputting the data in the area designated by a finger to the computer, a see-through type keyboard is a well known. It is very important consideration for such a touch keyboard to accurately and stably sense the touch condition and therefore a variety of sensing methods are proposed. One of such methods is the measurement of change of capacitance between the touch sensing electrode and the earth or ground occurring by finger touch. This technique is described in the U.S. Pat. No. 3,482,241 or U.S. application Ser. No. 437,220 filed Oct. 28, 1982.
The former describes a method of measuring change of capacitance of the touch sensing electrode by utilizing an AC bridge circuit, while the latter describes a method of measuring it by utilizing the resonant phenomenon in the resonant circuit.
The principle of a method of measuring change of capacitance of the touch sensing electrode by utilizing resonant phenomenon of the resonant circuit is briefly described hereunder. This method utilizes the fact that the resonant condition of resonant circuit changes in accordance with change of capacitance. Namely, an oscillator circuit which generates an AC signal of the predetermined frequency is connected to the touch sensing electrode and the resonant circuit which tunes to the frequency of said AC signal is connected between the touch sensing electrode and the earth. With such structure, the resonant condition of resonant circuit is changed in accordance with the touch/non-touch conditions to the touch sensing electrode and thereby an output voltage at the connecting point of the oscillating circuit and resonant circuit is changed.
However, in the existing method for detecting the touch condition to the touch sensing electrode by measuring change of capacitance at such touch sensing electrode, any fluctuation of capacity between mutual touch sensing electrodes is not considered. Therefore, resonant condition of resonant circuit is different in accordance with each touch sensing electrode to be sensed resulting in the disadvantage that an accurate and stable sensing result cannot be obtained and a resultant input error is caused.
The similar disadvantage is observed not only in the method of measuring change of capacitance by utilizing the resonant phenomenon of resonant circuit but also in the method of measuring change of capacitance by utilizing the AC bridge circuit.
Considered as the causes of different capacitance of each touch sensing electrode are error of touch sensing electrode occurring at the time of production, difference of length of the lead connected to the touch sensing electrode and fluctuation of floating capacity between the touch sensing electrodes.